74 II. A. GLEASON VEGETATIONAL HISTORY OF MIDDLE WEST 



so that races of similar ultimate ancestry but of different recent history 

 are again growing together. 



The Southern Migration. — The southern migration was participated 

 in chiefly by oaks ..and hickories of the drier uplands, and by oaks, 

 elms, ashes, walnuts, maples, hackberry, cottonwood, honey locust, 

 coffee tree, buckeye, and sycamore of the lowlands. Following as 

 closely behind the prairies as environmental change would permit, 

 these plants established two successional series, a xerarch series on 

 the uplands, leading to the establishment of an oak-hickory forest, and 

 a hydrarch series along the stream valleys, leading to the develop- 

 ment of marginal belts of hydrophytic or mesophytic forest. The 

 hydrarch series migrated more rapidly up the stream courses, since 

 the species concerned could take advantage of local conditions of favor- 

 able moisture, while the xerarch series found its most favorable route 

 along the broken topography of the river bluffs, between the prairies 

 above and the hydrarch forests below. *lThe two types of forest thus 

 advanced upon the prairies together along the river courses leading 

 southward into the Ohio valley and eastward into the Mississippi, and 

 the present outposts of this migration are still to be seen in Kansas, 

 Nebraska, the Dakotas, and Manitoba on the west and in Michigan 

 and Wisconsin on the north. 



Some of the mesophytes were left far behind in this advance, and 

 the beech, sugar maple, tulip tree, and hasswood are not particularly 

 important members of the climax forest or even totally absent from 

 Illinois west. In such cases a temporary climax developed in ravines 

 and on floodplains and was composed chiefly of a selection of species 

 persisting from the earlier successional stages. The slower trees have 

 been migrating, nevertheless, and with the greater success the farther 

 east. Beech is common in the Ohio valley: it has scarcely moved up 

 the Mississippi, but has followed the Wabash well into Indiana, and 

 the Miami, Muskingum, and Scioto until Ohio is occupied by it com- 

 pletely. There is reason to believe 48 that large areas of forest were 

 developed on the uplands in which the most mesophytic tree was the 

 red oak, Quercus rubra. 



The Northern Migration. — The northern migration proceeded from 

 the deciduous forests which had had a glacial center of preservation 

 in the Allegheny Mountains from Pennsylvania southward. These 

 species, now constituting the well known Alleghenian element in the 

 flora of New York and New England, have been during and since the 



4 8Gleason, H. A. An isolated prairie grove and its phvtogeographical significance. 

 Bot. Gaz. 53: 38-49. 1912. 



